Epidemiological studies on the association of nonprotein-bound estradiol levels in serum from women who developed breast cancer provide the rationale for our basic investigation of the formation of biologically activated metabolites of estradiol in estrogen-mediated cancer. breast cancer is considered to be hormonally mediated, and estrogens are believed to be the prime agents in tumor expression. The objective of this research program is to establish the role of catechol estrogen formation in estrogen-mediated cancer. Our working hypothesis is that the carcinogenic potential of estrogens will correlate with their rates of metabolic activation to catechols, but will not be related to their hormonal activity. Support for this hypothesis is provided by results from the in vitro neoplastic transformation of a subclone of Balb/c 3T3 cells and from an in vivo assay for hepatocellular carcinogenicity in Syrian hamsters. A group of four structurally related compounds composed of two hormonally active estrogens and two hormonally inactive derivatives will provide evidence to support our central hypothesis. A significant contribution of this program will be the discovery of a hormonally active noncarcinogenic estrogen. If our in vitro results are supported by evidence from the in vivo model system, it will provide confirmation of a suspected mechanism involved in carcinogenesis associated with endogenous nonprotein bound serum estradiol levels and the effects of therapeutic estrogens.